The present invention relates to a process for polymerizing dienes in a solvent comprising vinylaromatic monomers and to a process for preparing high-impact polystyrene, acrylonitrile-butadiene-styrene copolymers or methyl methacrylate-butadiene-styrene copolymers.
Various continuous and batchwise, solution or suspension processes are known for preparing high-impact polystyrene. In these processes, a rubber, usually polybutadiene, is dissolved in monomeric styrene which has been polymerized in a prereaction to a conversion of about 30%. The formation of polystyrene and the simultaneous decrease in the concentration of monomeric styrene results in a change in the phase coherence. During this event, known as xe2x80x9cphase inversionxe2x80x9d, grafting reactions occur on the polybutadiene, and these together with the intensity of stirring and the viscosity influence the nature of the dispersed soft phase. In the subsequent main polymerization, the polystyrene matrix is built up. Such processes carried out in various types of reactor are described, for example, in A. Echte, Handbuch der technischen Polymerchemie, VCH Verlagsgesellschaft Weinheim 1993, pages 484-89 and in U.S. Pat. Nos. 2,727,884 and 3,903,202.
In these processes, the separately prepared rubber has to be laboriously broken up and dissolved and the resulting polybutadiene rubber solution in styrene has to be filtered prior to the polymerization in order to remove gel particles.
Various attempts have therefore been made to prepare the necessary rubber solution in styrene directly by anionic polymerization of butadiene or butadiene/styrene in nonpolar solvents, for example cyclohexane or ethylbenzene, and subsequent addition of styrene (GB 1 013 205, EP-A-0 334 715 and U.S. Pat. No. 4,153,647) or by incomplete reaction of butadiene in styrene (EP-A 0 059 231, EP-A 0 304 088). The block rubber prepared in this way either has to be purified by precipitation or else the solvent and other volatile substances, in particular monomeric butadiene, have to be distilled off. In addition, owing to the high solution viscosity, only relatively dilute rubber solutions can be handled, which leads to high solvent, purification and energy costs.
U.S. Pat. No. 3,299,178 describes a process for polymerizing butadiene in styrene solution in the presence of a catalyst system comprising aluminum alkyls and titanium tetraiodide or titanium tetrachloride and iodine. However, a high halogen content leads to corrosion problems in the further processing of the rubber solution. The catalyst therefore has to be removed, which costs money.
WO 98/07765 and WO 98/07766 describe anionic polymerizations of butadiene in styrene. The anionic polymerization of styrene proceeds very quickly. For this reason, alkyls of alkaline earth metals, zinc and aluminum were used as retardant additives. The additives described have a retarding effect both on the polymerization of butadiene and on the polymerization of styrene. Without addition of randomizers, the anionic polymerization of a monomer mixture of styrene and butadiene initially gives a virtually pure homopolybutadiene block. To slow the subsequent styrene polymerization to a rate which can readily be controlled in industry, the additives have to be added in such amounts that the polymerization rate of butadiene is reduced too much and the overall process becomes uneconomical.
It is an object of the present invention to find a process for polymerizing dienes in a solvent comprising vinylaromatic monomers which leads to diene rubbers having a low incorporation of vinylaromatic monomers together with high butadiene conversions. The diene rubber solutions should as far as possible be able to be used directly, i.e. without removal of catalyst residues, degassing of unreacted dienes or precipitation of the diene rubber, for preparing impact-modified styrene polymers.
We have found that this object is achieved by a process for polymerizing dienes in a solvent comprising vinylaromatic monomers, wherein the polymerization is carried out in the presence of
a) an organometallic compound of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W as catalyst,
b) a Lewis acid as cocatalyst,
c) if desired, an alkylating agent and
d) if desired, a regulator.
The solvent preferably consists of from 85 to 100% by weight of vinylaromatic monomers and from 0 to 15% by weight of toluene, cyclohexane, methylcyclohexane, ethylbenzene or DECALIN(copyright) (decahydronaphthalene).
As catalyst, preference is given to using a metal chelate complex of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W having at least one chelating ligand bound via two O atoms, two N atoms or one O atom and one N atom. The chelate complex preferably has one or two chelating ligands. Preferred catalysts are metal chelate complexes of the formula (I), (II) or (III) 
where
R1, R2, R3, R4 and R5 are, independently of one another, hydrogen, unsubstituted or halogen- or alkoxy-substituted C1-C20-alkyl, C7-C20-arylalkyl, C6-C22-aryl or alkylaryl, C3-C15-cyoloalkyl or alkylcycloalkyl, C1-C20-alkoxy, C3-C15-cycloalkoxy, C6-C22-aryloxy or together with one or more of C1, C2, C3, O and N form a monocyclic, bicyclic or heterocyclic, aliphatic or aromatic ring system having from 3 to 15 ring atoms,
M is Ti, Zr, Hf. V, Nb, Ta, Cr. Mo, W.
X is an anion,
D is an uncharged donor ligand,
n is 1, 2, 3 or 4,
m is the valence of M,
k is an integer or fraction in the range from 0 to 10.
R1, R2, and R3 can be, independently of one another, hydrogen, methyl, i-propyl, t-butyl, cyclohexyl, methoxy, ethoxy, i-propoxy, t-butoxy, cyclohexoxy, phenyl, 2,6-di-tert-butylphenyl, 2,6-di-tert-butyl-4-methylphenyl, phenoxy, 2,6di-tert-butylphenoxy, 2,6-di-tert-butyl-4-methylphenoxy and R4 and R5 can be, independently of one another, hydrogen, methyl, i-propyl, t-butyl, cyclohexyl, phenyl, 2,6-diisopropylphenyl, 2,6-di-tert-butylphenyl, 2,6-di-tert-butyl-4-methylphenyl.
Examples of suitable anions X are unsubstituted or halogen- or alkoxy-substituted C1-C20-alkoxides, C3-C15-cycloalkoxides, C6-C22-arylalkoxides, tetraalkylsilyloxy, dialkylamide, bis(trialkylsilyl)amide, halide, sulfonates such as triflates, cyanides, substituted or unsubstituted dialkyl or diaryl phosphates having from 1 to 20 carbon atoms.
Suitable donor ligands D in the formulae (I) to (III) are, for example, tetrahydrofuran, diethyl ether, pyridine, dioxane, tetraxnethylenediamine or triethylamine.
As chelating ligands, it is possible to use, for example, 2,2,6,6-tetramethylheptane-3,5-dione, 2,2,4,6,6-pentamethylheptane-3,5-dione, 1,3-diphenylpropane-1,3-dione, 1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)propane-1,3-dione, 3-phenylpentane-2,4-dione, 1,1,1-trifluoro-4-naphthylbutane-2,4-dione, 3-heptafluorobutyryl-(+)camphor, 1,1,1,2,2-pentafluoro-6,6-dimethylheptane-3,5-dione, tert-butyl salicylate, di-tert-butyl malonate, di(2,6-di-tert-butyl-4-methylphenyl)malonate in deprotonated form.
Particular preference is given to using a titanium compound as catalyst, in particular bis(2,2,6,6-tetramethyl-3,5-heptanedionato)Ti(IV) dichloride, (2,2,6,6-tetramethyl-3,5-heptanedionato)Ti(III) dichloride (THF adduct), (2,2,6,6-tetramethyl-3,5-heptanedionato)Ti(IV) triisopropoxide, (2,2,6,6-tetramethyl-3,5-heptanedionato)Ti(III) diisopropoxide (THF adduct), bis(2,2,6,6-tetramethyl-3,5-heptanedionato)Ti(IV) diisopropoxide or bis(1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedionato)Ti(IV) diisopropoxide.
Particularly preferred complexes of the formula (I) are bis(2,2,6,6-tetramethyl-3,5-heptanedionato)titanium(IV) diisopropoxide and bis(2,2,6,6-tetramethylheptane-3,5-dionato)titanium(IV) dichloride.
Preference is given to using a cocatalyst which contains no ionically bound halogen. Examples of suitable cocatalysts are open-chain or cyclic aluminoxane compounds such as methylaluminoxane (MAO), boranes, preferably phenylboranes substituted by fluorine or fluoroalkyl groups, e.g. tris(pentafluorophenyl)borane, and borates such as tetrakis(pentafluorophenyl)borate of noncoordinating counterions, preferably N,N-dimethylanilinium.
If desired, the polymerization can be carried out in the presence of not only the catalyst and cocatalyst but also an alkylating agent and/or a regulator.
Alkylating agents which can be used are alkyl compounds of aluminum, magnesium or zinc bearing at least one C1-C20-alkyl group. Preference is given to using a monoalkyl, dialkyl or trialkyl compound of aluminum, for example triethylaluminum or triisobutylaluminum, as alkylating agent.
Possible regulators are hydrogen, aluminum hydrides such as dibutylaluminum hydride, olefins such as ethylene, propylene, butylene, 1-hexene or 1-octene, allyl compounds such as allylmagnesium bromide, bisallyl magnesium, phenylacetylene, trimethylvinylsilane, allyl ethers, allyl halides or allyl esters or polar comonomers such as methyl, ethyl, butyl, hexyl or glycidyl acrylates or methacrylates, vinyl acetate, vinylimidazole, vinylpyrrolidone, vinyl ethers, in particular ethyl or isobutyl vinyl ether or acetonitrile. The regulator allows the viscosity of the rubber solution to be adjusted.
The catalyst is generally used in a molar ratio of from 1;100,000 to 1:1000, based on the metal of the catalyst to the diene.
The molar ratio of catalyst to cocatalyst is generally from 1:0.5 to 1:10,000. in the case of organoaluminum compounds as cocatalyst preferably from 1:10 to 1:10,000, particularly preferably from 1:20 to 1:2000, in particular from 1:30 to 1:1000. In the case of boron compounds as cocatalyst, the molar ratio of catalyst to cocatalyst is preferably from 1:0.9 to 1:2.
If an alkylating agent is used, the molar ratio of alkylating agent to catalyst is generally from 0.3:1 to 10,000:1. The regulator is generally added in amounts which result in a molecular weight of the rubber in the range from 100,000 to 500,000 g/mol. in particular from 300,000 to 400.000 g/mol. As regards these molecular weights, the viscosities of the rubber solutions are in the industrially handleable region. The molar ratio of regulator to catalyst is generally from 0.5:1 to 10,000:1.
If an alkylating agent is used, it is preferably premixed with the catalyst and after from about 10 to 60 minutes is combined with the monomers containing the cocatalyst and any regulator used.
Preferred monomers are 1,3-butadiene, isoprene, 2,3-dimethylbutadiene, 1,3-pentadiene, 1,3-hexadiene or piperylene or mixtures thereof.
As comonomers and as xe2x80x9csolventxe2x80x9d or xe2x80x9csolvent componentxe2x80x9d in the process of the present invention, suitable vinylaromatic compounds are, for example, styrene, xcex1-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene, vinyltoluene or 1,1-diphenylethylene or mixtures thereof. Styrene is particularly preferred as xe2x80x9csolventxe2x80x9d or xe2x80x9csolvent componentxe2x80x9d.
For practical reasons, a small amount of a further solvent can be used. Suitable further solvents are aliphatic, cycloaliphatic or aromatic hydrocarbons having from 4 to 12 carbon atoms, e.g. pentane, hexane, heptane, cyclohexane, methylcyclohexane, isooctane, benzene, alkylbenzenes such as toluene, xylene, ethyl benzene and DECALIN(copyright) (decahydronaphthalene) and suitable mixtures. If a further solvent is used, the amount of vinylaromatic compound as a proportion of the total amount of solvent is generally in the range from 85 to 100 percent by weight, preferably from 90 to 100 percent by weight.
The starting concentration of the diene is generally in the range from 3 to 20 percent by weight, preferably from 5 to 10 percent by weight, based on the total solution.
The polymerization of the dienes is generally carried out at from 0 to 100xc2x0 C., preferably from 20 to 60xc2x0 C.
After reaching the desired conversion, the polymerization can be stopped by deactivation of the catalyst using protic compounds, for example water, carboxylic acids and/or alcohols.
The process of the present invention gives polydienes having a relatively low proportion of copolymerized vinylaromatic monomers. The content of 1,2-vinyl linkages is generally in the range from 1 to 25 mol %, preferably from 2 to 10 mol %, and the content of 1,2-trans linkages is generally less than 50 mol %, so that an undesirable tendency of the product to crystallize can be avoided.
If desired, gel inhibitors such as hydrocarbon halides, silicon halides and 1,2-diolefins can also be used. The amounts used depend on the compound used in the individual case. The preferred 1,2-butadiene is generally used in amounts of from about 100 to 3000 ppm.
The diene rubber obtained by the process of the present invention can in principle be used in all processes in which use is made of rubber solutions in vinylaromatic compounds, which are usually obtained by dissolving the rubber in the vinylaromatic compound and/or additional solvents. The rubber solutions can, if desired after addition of further monomers, including ethylenically unsaturated compounds other than vinylaromatic compounds, solvents and/or initiators, be subjected directly to an anionic polymerization or a free-radical polymerization initiated thermally or by means of free radical initiators.
The rubber solutions are particularly suitable for producing molding compositions comprising vinylaromatic monomers, for example high-impact polystyrene (HIPS), acrylonitrile-butadiene-styrene polymers (ABS) and methyl methacrylate butadiene-styrene copolymers (MBS).
As monomers for forming the matrix, further ethylenically unsaturated compounds, in particular aliphatic vinyl compounds such as acrylonitrile, acrylic or methacrylic esters, for example the methyl, ethyl, ethylhexyl or cyclohexyl esters, maleic esters, maleic anhydride or maleiimide, as well as the abovementioned vinylaromatic compounds can be added to the diene rubber solution.
Suitable free radical initiators are peroxides, for example diacyl, dialkyl and diaryl peroxides, peroxyesters, peroxydicarbonates, peroxyketal, peroxosulfates, hydroperoxides or azo compounds. Preference is given to using dibenzoyl peroxide, 1,1-di-tert-butylperoxocyclohexane, dicumyl peroxide, dilauryl peroxide and azobisisobutyronitrile.
As auxiliaries, it is possible to add molecular weight regulators such as dimeric xcex1-methylstyrene, mercaptans such as n-dodecyl mercaptan or tert-dodecyl mercaptan, chain branching agents, stabilizers or mold release agents.
The polymerization of the matrix can be carried out entirely in bulk or in solution. The polymerization is generally carried out at from 50 to 200xc2x0 C., preferably from 90 to 150xc2x0 C., in the case of a free-radical polymerization or in the range from 20 to 180xc2x0 C., preferably from 30 to 80xc2x0 C., in the case of an anionic polymerization. The reaction can be carried out isothermally or adiabatically.
The process of the present invention offers the advantage that the molding compositions can be produced in a xe2x80x9csingle-vessel reactionxe2x80x9d without laborious changes of reaction medium. In addition, no solvent or only small amounts of solvents are required, so that their costs and the costs of purification and work-up are largely eliminated.
In a particular embodiment, the preparation of the rubber solution and the polymerization of the matrix are carried out in one continuous process. For this purpose, for example, the diene rubber required for formation of the soft phase is polymerized as described above in a first reaction zone and is passed directly to a second reaction zone. In this second reaction zone, further vinylaromatic or olefinic monomers may be added in an amount sufficient to achieve phase inversion and, if desired, further anionic or free radical initiators and, if desired, solvents may also be added and the mixture is polymerized to phase inversion. In a third reaction zone, the polymerization with that amount of vinylaromatic or olefinic monomer necessary to form the impact-modified thermoplastic molding composition is completed anionically or by a free radical mechanism.
The molding compositions obtained can be freed of solvents and residual monomers by means of degassers or vented extruders at atmospheric pressure or under reduced pressure and temperatures of from 190 to 320xc2x0 C.
If the matrix of the rubber-modified molding composition is also built up by anionic polymerization, it can be advantageous to crosslink the rubber particles by appropriate temperature conditions and/or by addition of peroxides, in particular those having a high decomposition temperature, e.g. dicumyl peroxide.